Method for recovering elemental silicon from silicon sludge by electrolysis in non-aqueous electrolyte

ABSTRACT

The present invention relates to a method for recovering elemental silicon from silicon sludge by electrolysis in a non-aqueous electrolyte. The recovery method of silicon according to the present invention can achieve direct reduction of silicon by electrolysis at a low temperature (below 200° C.), control the structure of silicon by a simple process and a change in electrolysis conditions, and perform a continuous process by adding a silicon salt.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean Patent Application No. 10-2012-133845, filed on Nov. 23, 2012, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to a method for recovering elemental silicon from silicon sludge by electrolysis in a non-aqueous electrolyte.

2. Discussion of Related Art

Cutting slurry containing silicon carbide of 20 μm in size and the like is used in a process of cutting wafers from a silicon ingot. In this process, sludge containing silicon, silicon carbide, metal powder, cutting oil, etc. is discharged and effective separation and recovery of waste sludge for reuse or recycling as a useful resource has a significant meaning in terms of efficient use of resources as well as in terms of environmental protection.

In general, the refinement of silicon can be obtained by reacting concentrated silicon dioxide (SiO₂) with a reductant such as carbon at high temperature, and the refined silicon has a purity of about 98% and thus has limitations on its use. Moreover, in the refinement process, a lot of energy is used, a large amount of carbon dioxide is emitted, and impurities are contained, which are very problematic. A purification process of silicon is necessary to use silicon in higher value-added industries such as semiconductor, photovoltaic, secondary battery industries. The purification of silicon yields a purity of 99.9% or higher by conversion into halogen compounds, fractional distillation, and reduction, but the process is complicated, energy-consuming, and uses toxic substances. However, no alternative process has yet been suggested.

Moreover, a thinning process is necessary to use silicon as a semiconductor or a solar cell, which requires the formation of wafers by an ingot cutting process after melting and solidification or a deposition method such as chemical vapor deposition or sputtering. This process is performed under high vacuum and is difficult to be carried out as a continuous process, which involves a lot of expenses.

Over the past few decades, extensive research has been conducted on processes for easily manufacturing silicon thin films, but there is no alternative yet. Electrolysis is a simple process that yields materials of desired properties by simple manipulation of voltage, current, etc. and is suitable for continuous mass production. However, the electrolytic reduction of silicon is very difficult to achieve in a typical aqueous electrolyte due to low oxidation-reduction potential and high reduction overpotential. Moreover, even a very small amount of reduced silicon is reoxidized with oxygen in an aqueous solution, and thus it is impossible to achieve the electrolytic reduction of elemental silicon.

According to literature search, a method for electrolytic reduction of silicon by electrolysis in a non-aqueous electrolyte has been studied. Cohen and Huggins prepared SiF₆ by electrolysis in a LiH-KF process (U. Cohen and R. A. Huggins, 1976: Silicon Epitaxial Growth by Electrodeposition from Molten Fluorides, J. Electrochem. Soc., 123, pp. 381-383), and research on monocrystalline and polycrystalline silicon started since then. Elwell et al. examined the possibility of continuous deposition of silicon thin films in LiF—KF—NaF—K₂SiF₆ in a molten state (D. Elwell and G. M. Rao, 1988: Electrolytic production of silicon, J. Appl. Electrochem., 18, pp. 15-22). They studied electrodeposition using a molten salt at high temperatures of 500 to 1,400° C., which requires a complicated process and a high temperature process, resulting in significant cost and energy. It was reported that silicon was reduced in high temperature molten salts using K₂SiF₆, Na₂SiF₆, SiO₂, etc. as a source of silicon for the electrolytic reduction of silicon in an aprotic solvent (A. K. Agrawal and A. E. Austin, 1981: Electrodeposition of silicon from solutions of silicon halides in aprotic solvents, J. Electrochem. Soc., 128, pp. 2292-296) or in a molten salt (K. L. Carleton, J. M. Olson, and A. Kibbler, 1983: Electrochemical nucleation and growth of silicon in molten fluorides, J. Electrochem. Soc., 130, pp. 782-786) and it was possible to form a uniform thin film up to 0.25 μm in thickness (J. Gobet and H. Tannenberger, 1986: Electrodeposition of silicon from a nonaqueous solvent, J. Electrochem. Soc., 133, pp. C322). Moreover, recently, Nicholson et al. reported that silicon could be electrodeposited in a low temperature molten salt using a non-aqueous solvent but the electrodeposited silicon had strong oxidative properties, resulting in the formation of silica (J. P. Nicholson, 2005: Electrodeposition of silicon from nonaqueous solvents, J. Electrochem. Soc., 152, pp. C795-802).

It was reported that the electrolytic reduction of highly active metals such as aluminum, titanium, magnesium, etc., which was impossible in an aqueous electrolyte, was possible using an ionic liquid having wide electrochemical window and excellent electrical conductivity as an electrolyte (F. Endres, 2002: Ionic Liquids: Solvents for the electrodeposition of metals and Semiconductors, Phys. Chem. Chem. Phys., 3, pp. 144-154). A highly conductive electrolyte for minimizing hydrogen generation using an organic solvent was studied by Austin (A. E. Austin, 1976: U.S. Pat. No. 3,990,953 and November 9 (1976) CA 86: 10098c). In general, the electrolyte was prepared using an aprotic solvent such as silicon halide, propylene carbonate (PC), or tetrahydrofuran (THF), and this electrolyte has no conductivity and thus requires a supporting electrolyte. In the study of Austin, silicon halide and tetrabutylammonium perchlorate (Bu₄NClO₄) as the supporting electrolyte were added to prepare various organic electrolytes, which was described by Bucker and Amick (E. R. Bucker and J. A. Amick, 1980: U.S. Pat. No. 4,192,720. October 16 (1980) CA 92: 10098). Recently, a study on electrolytic reduction of silicon using PC was reported by Fukunaka et al. (Y. Nishimura and Y. Fukunaka, 2007: Electrochemical reduction of silicon chloride in a non-aqueous solvent, Electrochimca Acta, 53, pp. 111-116), but as a result of XPS analysis, the electrodeposited silicon was exposed to the air and oxidized, and Munisamy et al. performed a study on the initial growth of silicon in acetonitrile by electrolysis (T. Munisamy and A. J. Bard, 2010: Electrodeposition of Si from organic solvents and studies related to initial stages of Si growth, Electrochimca Acta, 55(11, 15), pp. 3797-3803). Abedin et al. reported that silicon was electrodeposited using a room temperature ionic liquid (S. Z. El Abedin, N. Borissenko, and F. Endres, 2004: Electrodeposition of nanoscale silicon in a room temperature ionic liquid, Electrochem. Comm., 6, pp. 510-514), and Mallet et al. reported that silicon nanowires were first synthesized by the same method (J. Mallet, M. Molinari, F. Martineau, F. Delavoie, P. Fricoteaux, and M. Troyon, 2008: Growth of silicon nanowires of controlled diameters by electrodeposition in ionic liquid at room temperature, Nano Lett. 8(10), pp. 3468-3474).

They all reported a study on the electrolytic reduction of silicon at room temperature but did not solve the problem that the reduced silicon is oxidized. Moreover, it is not yet determined whether the electrodeposited silicon oxide is reduced along with dissolved oxygen in the electrolyte or whether the electrodeposited porous silicon is exposed to the air and oxidized. Furthermore, a study aimed at stabilizing the electrodeposited silicon by annealing before being exposed to the air has been partially performed, but this problem has not yet been solved.

Thus, there is an urgent need to develop electrolysis conditions in the recovery of elemental silicon from silicon sludge by electrolysis that is simpler than the conventional processes.

In this study, cutting oil, metal impurities, etc. were removed from silicon sludge by a separation process such as mechanical separation to separate silicon, and a mixture of the separated silicon and silicon carbide was subjected to chloridizing roasting to prepare silicon tetrachloride. The silicon tetrachloride was dissolved in a conductive non-aqueous solvent, and elemental silicon was directly reduced by electrolysis. The crystallinity and composition of electrodeposits and the presence of impurities were determined from the analysis of the finally produced electrodeposits.

SUMMARY OF THE INVENTION

The present inventors have studied on a method for recovering silicon from silicon sludge by electrolysis and found that elemental silicon could be directly reduced in a conductive non-aqueous electrolyte prepared from silicon tetrachloride and, and through subsequent heat treatment, the efficiency of electrolysis of silicon and the stabilizing efficiency of silicon could be significantly improved, thus completing the present invention.

Accordingly, an object of the present invention is to provide a method for recovering elemental silicon from silicon sludge by electrolysis in a non-aqueous electrolyte, in which silicon tetrachloride simply prepared from silicon sludge is dissolved, which can easily control electrolysis parameters to control the structure of silicon and easily recover the elemental silicon using the non-aqueous electrolyte.

To achieve the above object, the present invention provides a method for recovering elemental silicon, the method comprising the steps of: (a) mixing waste silicon sludge and an organic solvent to separate cutting oil from the silicon sludge; (b) separating iron (Fe) from the silicon sludge, from which the cutting oil is removed, using a magnetic separator; (c) adding chlorine to the silicon sludge, from which the iron is removed, and heating the resulting silicon sludge to prepare silicon tetrachloride (SiCl₄); (d) placing an electrolytic cell provided with conductive electrodes in a conductive non-aqueous solvent, in which the silicon tetrachloride is dissolved, in a high-purity inert gas atmosphere; and (e) applying an electric power to the electrolytic cell such that a reduction of silicon occurs in a negative electrode and a silicon thin film is formed on the surface of the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram showing a method for preparing silicon tetrachloride from silicon sludge;

FIG. 2 is a diagram showing the analysis results of solid components, obtained by separate cutting oil and iron powder from silicon sludge, using X-ray diffraction and electron microscopy;

FIG. 3 is a diagram showing electrochemical behaviors of silicon in an [EMIM]TFSI electrolyte, in which 0.1 M silicon tetrachloride is dissolved, measured by cyclic voltammetry using gold as an electrode;

FIG. 4 is a diagram showing the results of SEM-EDS of silicon reduced in a gold electrode;

FIG. 5 is a diagram showing the results of XPS measurement of silicon recovered by the present invention; and

FIG. 6 is a diagram showing the results of XRD measurement of silicon recovered by the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, the present invention will be described in detail.

The present invention provides a method for recovering elemental silicon, the method comprising the steps of: (a) mixing waste silicon sludge and an organic solvent to separate cutting oil from the silicon sludge; (b) separating iron (Fe) from the silicon sludge, from which the cutting oil is removed, using a magnetic separator; (c) adding chlorine to the silicon sludge, from which the iron is removed, and heating the resulting silicon sludge to prepare silicon tetrachloride (SiCl₄); (d) placing an electrolytic cell provided with conductive electrodes in a conductive non-aqueous solvent, in which the silicon tetrachloride is dissolved, in a high-purity inert gas atmosphere; and (e) applying an electric power to the electrolytic cell such that a reduction of silicon occurs in a negative electrode and a silicon thin film is formed on the surface of the electrode.

In the recovery method of silicon according to the present invention, step (a) is to separate cutting oil from silicon sludge, in which an organic solvent miscible with the cutting oil is mixed with waste sludge to selectively dissolve the cutting oil, thus separating the cutting oil from the silicon sludge.

The organic solvent is preferably, but not limited to, chloroform, ethyl acetate, tetrahydrofuran (THF), or dichloromethane (CH₂Cl₂).

Step (b) is to remove iron (Fe) powder contained in the silicon sludge from which the cutting oil is removed. The iron powder is generated by friction between wire saw and silicon and is separated using a magnetic separator (Eric manufacturing). The magnetic separator preferably has a magnetic flux density of 500 gauss.

Step (c) is to prepare silicon tetrachloride (SiCl₄) from the silicon sludge after the iron is separated therefrom. The silicon sludge from which the iron is removed is a mixture of silicon and silicon carbide, and chlorine is added to the silicon sludge and heated to recover the silicon tetrachloride. The heating is preferably performed at 800 to 1,200° C. for 30 to 90 minutes.

In step (d), the basic structure of the electrolytic cell for electrolytic reduction of silicon is a three-electrode system in which a reference electrode is additionally used in an electrolytic cell comprising a working electrode and a counter electrode.

The electrodes may be made of any conductive material and preferably comprise at least two selected from the group consisting of gold, platinum, and copper, but not limited thereto.

In step (d), the high-purity inert gas may comprise at least one selected from the group consisting of nitrogen, helium, argon, neon, and xenon, and preferably argon. When the silicon is recovered in a high-purity inert gas atmosphere, it is possible to prevent oxidation of silicon.

In step (d), a conductive non-aqueous solvent capable of dissolving the silicon tetrachloride may be used as an electrolyte. The conductive non-aqueous solvent is a conductive non-aqueous solvent containing a bis(trifluoromethylsulfonyl)imide (TFSI) anion and comprises, but not limited to, at least one selected from the group consisting of (1-Butyl-3-methyl-pyridinium bis(trifluoromethylsulfonyl)imide) [BMPy]TFSI, (1-methyl-propylpiperidinium bis(trifluoromethylsulfonyl)imide) PP13TFSI, and (1-Ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide) [EMIM]TFSI.

Moreover, the conductive non-aqueous solvent may further comprise propylene carbonate (PC), dichloromethane (DCM), tetrahydrofuran (THF), dicyanamide (DCA), or N-methylpyrrolidone (NMP). These materials have very low viscosity and have no effect on the dissociation of silicon, but improve the electrical conductivity of the electrolyte.

The method of the present invention may further comprise, before step (d), the steps of (d′) cleaning the electrodes and the cell with a mixture solution of sulfuric acid (H₂SO₄) and hydrogen peroxide (H₂O₂); and (d″) drying the non-aqueous solvent at 80 to 120° C. for 20 to 30 hours. In step (d′), the sulfuric acid and the hydrogen peroxide may preferably be mixed in a volume ratio of 50:50, and through this step, the impurities can be removed from the electrodes and the cell. Moreover, through step (d″), a small amount of water contained in the non-aqueous solvent can be removed.

Step (e) is to apply an electric power to the electrolytic cell such that a reduction of silicon occurs in a negative electrode and a silicon thin film is formed on the surface of the electrode. As used in typical electroplating, when two metal electrodes are placed in an electrolyte in which silicon is dissolved and an electric power is applied thereto, a reduction of silicon occurs in the negative electrode, and silicon electrodeposits are formed on the surface of the electrode.

The method of the present invention may further comprise, after step (e), the step of performing heat treatment in an inert gas atmosphere at 800 to 900° C. for 30 to 90 minutes. The inert gas may comprise, but not limited to, nitrogen, helium, argon, neon, and xenon. With the heat treatment, it is possible to remove impurities from the silicon and stabilize the electrodeposited silicon.

As mentioned above, when the silicon is recovered by the recovery method according to the present invention, it is possible to directly obtain elemental silicon by the electrolysis, which is easier than the conventional processes. Moreover, the silicon can be recovered at low temperature, and thus it is possible to allow mass production and reduce the production cost. Furthermore, the recovery method of silicon according to the present invention can prepare the silicon tetrachloride by a simple process and continuously recover the silicon by repeatedly dissolving the silicon tetrachloride.

Next, preferred examples will be provided for better understanding of the present invention. However, the following examples are provided only for the purpose of illustrating the present invention, and the scope of the present invention is not limited by the examples.

Example 1 Preparation of Silicon Tetrachloride from Silicon Sludge

The outline of a process for preparing silicon tetrachloride as a pre-treatment for recovering silicon from silicon sludge is shown in FIG. 1.

As an organic solvent miscible with cutting oil in silicon sludge, dichloromethane (CH₂Cl₂) was mixed with silicon sludge and stirred at 300 rpm or higher for 3 hours to selectively dissolve the cutting oil in the silicon sludge. After dissolving the cutting oil, silicon and silicon carbide (Si+SiC) as a solid phase and organic oil as a liquid phase were separated by filtration and centrifugation. Iron powder contained in the solid phase mixture, from which the organic oil is separated, was separated using a magnetic separator (Eric manufacturing) having a magnetic flux density of 500 gauss. The resulting mixture was stirred in 1 mol/L of hydrochloric acid solution at a solid-liquid ratio of 1:2 at room temperature for 2 hours, and the solid phase was precipitated, washed with distilled water, and then dried. The solid components from which the iron powder was separated were analyzed using X-ray diffraction and electron microscopy, and the results are shown in FIG. 2.

As shown in FIG. 2, the amount of iron removed was less than 0.1 wt % with respect to the total solid content, and thus a mixture of silicon and silicon carbide was confirmed.

Then, 5 wt % of carbon powder was added to the separated and concentrated mixture of silicon and silicon carbide, and the resulting mixture was placed in a furnace. Subsequently, chlorine gas was injected into the furnace, and gas vaporized by chloridizing roasting at 1,000° C. for 1 hour was condensed at 25° C., thus preparing silicon tetrachloride (SiCl₄).

Example 2 Electrolytic Recovery of Silicon from Silicon Tetrachloride

1. Experimental Conditions for Electrolytic Recovery of Silicon

A three-electrode system was used as an electrolytic cell for electrolytic reduction of silicon. [EMIM]TFSI in which silicon tetrachloride was dissolved was used as an electrolyte. To prevent oxidation of silicon, the experiment was performed in a glove box in which high-purity argon gas (5 N, oxygen content of no more than 1 ppm, and vapor content of no more than 3 ppm) was filled. The effects of the stable voltage window of an ionic liquid and the electrolysis conditions (electrodes, ionic liquid, silicon concentration, etc.) on the electrochemical properties were measured by cyclic voltammetry using a Potentiostat/Galvanostat (Solartron 1287). At this time, the potential range was −4 to 2 V (vs. OCV), and the scan rate was 10 mV/s.

The electrochemical oxidation/reduction behaviors of silicon when gold (Ag) was used as a working electrode were examined. A platinum (Pt) wire was used as a counter electrode (4 cm²) and a quasi reference electrode (QRE), respectively. To remove impurities, all electrodes and cells were cleaned with a mixture solution of sulfuric acid (H₂SO₄) and hydrogen peroxide (H₂O₂) in a volume ratio of 50:50 and then used in the experiment. Moreover, all ionic liquids were dried in a vacuum oven at 100° C. for 24 hours prior to the experiment to remove a small amount of water contained therein and then used in the electrochemical experiment. To analyze the morphology, composition, and crystallinity of elemental silicon obtained by electrolysis, an experiment was performed to reduce elemental silicon directly from [EMIM]TFSI, in which 0.5 M silicon tetrachloride was dissolved, under potentiostatic electrolysis (−1.9 V vs. Pt-QRE).

2. Electrolytic Recovery of Silicon

Electrochemical behaviors of silicon in an [EMIM]TFSI electrolyte with a gold electrode are shown in FIG. 3.

As shown in FIG. 3, the stable electrochemical window was about 3 V, and the cathodic potential limit was −2.5 V (vs. Pt-QRE). In an electrolyte in which 0.5 M SiCl_(a) was dissolved, a strong reduction current, expected as a stable reduction of silicon in view of the cathodic potential limit, was observed. The cyclic voltammetric behaviors in an electrolyte in which silicon tetrachloride (SiCl₄) was dissolved showed a first reduction zone where the reduction current increased from 0 V and a second reduction zone where the reduction current density sharply increased at −2.2 V. It is considered that tetravalent silicon ions are changed to other compounds in the first reduction zone and that ionic silicon and intermediate silicon compounds are reduced to silicon in the second reduction zone starting from −2.2 V. In particular, it was found that since the peak, expected as the reduction current of silicon, was at a higher potential than the reduction potential limit, the possibility that impurities were contained in the reduction reaction was low.

Experimental Example 1 Analysis of Silicon Electrodeposited Film

1. Analysis Method

The morphology and composition of the electrodeposited silicon film were analyzed using a field-emission scanning electron microscope (FE-SEM, JSM-6500F) with an energy-dispersive spectrometer (EDS) attached and X-ray diffractometer (XRD). Impurities in the electrodeposited silicon film were analyzed using X-ray photoelectron spectroscopy (XPS, ULVAC-PHI, Quantera SXM), and a depth analysis for a depth of about 50 nm from the surface was performed along with surface analysis. Moreover, the crystallinity of silicon was analyzed using X-ray diffractometer (XRD, Rigaku D/MAX-200-, Cu-Ka).

2. Analysis Results of Electrodeposited Film

The results of SEM-EDS of silicon reduced in a gold electrode are shown in FIG. 4.

As shown in FIG. 4, the morphology of silicon is in the form of particles of about 100 nm, and the results of the EDS analysis show the silicon reduced along with gold as the working electrode.

Since the silicon was obtained in the stable region confirmed from the cyclic voltammetric curve, other impurities than the working electrode were not observed, but a large amount of oxygen was detected. Since the experiment for measuring the reduction of silicon and the properties of ionic liquids was performed in an argon-filled glove box, the possibility that the silicon was reduced to oxide was low. Moreover, since the ionic liquids were dried in a vacuum oven at 100° C. for more than 24 hours prior to the experiment to remove moisture therein, it was determined that the detected oxygen was generated in the surface while the prepared samples were exposed to the outside during transfer to the analysis instruments. The results of XPS measurement for the analysis of the surface and impurities are shown in FIG. 5.

As shown in FIG. 5, the amount of reduced silicon was very low as 31.3%, and other impurities than oxygen were not observed. The silicon was detected in the form of SiO₂, and the depth analysis was performed to determine whether the silicon surface was changed or whether the silicon was reduced to oxide. From the results of analysis at a depth of 20 nm from the surface, it was found that the intensity of SiO₂ peak was reduced from the surface to the interior and a Si peak was observed. A complete Si peak was observed at a depth of more than 20 nm, from which it was confirmed that SiO₂ was not generated during the electrolytic reduction but only the surface was oxidized when the samples were exposed to the outside for the analysis. Moreover, the components of the electrolyte and chloride ions were not detected, from which it was confirmed that pure silicon was reduced.

The results of XRD measurement for the analysis of the crystallinity of the electrolytically reduced silicon are shown in FIG. 6. The silicon peaks were analyzed based on the peaks of gold used as the working electrode, and several silicon crystal faces were observed, from which it was confirmed that silicon was present in the form of polycrystalline or amorphous silicon.

As described above, the recovery method of silicon according to the present invention can achieve direct reduction of silicon by electrolysis at a low temperature (below 200° C.), control the structure of silicon by a simple process and a change in electrolysis conditions, and perform a continuous process by adding a silicon salt.

It will be apparent to those skilled in the art that various modifications can be made to the above-described exemplary embodiments of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers all such modifications provided they come within the scope of the appended claims and their equivalents. 

What is claimed is:
 1. A method for recovering elemental silicon, the method comprising the steps of: (a) mixing waste silicon sludge and an organic solvent to separate cutting oil from the silicon sludge; (b) separating iron (Fe) from the silicon sludge, from which the cutting oil is removed, using a magnetic separator; (c) adding chlorine to the silicon sludge, from which the iron is removed, and heating the resulting silicon sludge to prepare silicon tetrachloride (SiCl4); (d) placing an electrolytic cell provided with conductive electrodes in a conductive non-aqueous solvent, in which the silicon tetrachloride is dissolved, in a high-purity inert gas atmosphere; and (e) applying an electric power to the electrolytic cell in step (d) such that a reduction of silicon occurs in a negative electrode and a silicon thin film is formed on the surface of the electrode.
 2. The method of claim 1, wherein in step (a), the organic solvent comprises chloroform, ethyl acetate, tetrahydrofuran (THF), or dichloromethane (CH₂Cl₂).
 3. The method of claim 1, wherein in step (b), the magnetic separator has a magnetic flux density of 500 gauss.
 4. The method of claim 1, wherein in step (c), the heating is performed at 800 to 1,200° C. for 30 to 90 minutes.
 5. The method of claim 1, wherein in step (d), the electrodes comprises at least two selected from the group consisting of gold, platinum, and copper.
 6. The method of claim 1, wherein in step (d) the inert gas comprises at least one selected from the group consisting of nitrogen, helium, argon, neon, and xenon.
 7. The method of claim 1, wherein in step (d), the conductive non-aqueous solvent is a conductive non-aqueous solvent containing a bis(trifluoromethylsulfonyl)imide (TFSI) anion and comprises at least one selected from the group consisting of (1-Butyl-3-methyl-pyridinium bis(trifluoromethylsulfonyl)imide) [BMPy]TFSI, (1-methyl-propylpiperidinium bis(trifluoromethylsulfonyl)imide) PP13TFSI, and (1-Ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide) [EMIM]TFSI.
 8. The method of claim 7, wherein the conductive non-aqueous solvent further comprises propylene carbonate (PC), dichloromethane (DCM), tetrahydrofuran (THF), dicyanamide (DCA), or N-methylpyrrolidone (NMP).
 9. The method of claim 1, further comprising, before step (d), the steps of: (d′) cleaning the electrodes and the cell with a mixture solution of sulfuric acid and hydrogen peroxide; and (d″) drying the non-aqueous solvent at 80 to 120° C. for 20 to 30 hours.
 10. The method of claim 1, further comprising, after step (e), the step of performing heat treatment in an inert gas atmosphere at 800 to 900° C. for 30 to 90 minutes.
 11. The method of claim 10, wherein the inert gas comprises at least one selected from the group consisting of nitrogen, helium, argon, neon, and xenon. 